The present invention is directed to magnetohydrodynamic sensing devices. More particularly the present invention is concerned with magnetohydrodynamic perturbation sensing apparatus of extremely shock resistant characteristics.
In the prior art a variety of seismological sensing apparatus has been proposed for monitoring various forms of disturbances or shock waves. These shock waves may be generated in response to earthquakes, tests of nuclear warheads, or from other conventional sources such as the hydraulic vibrators or conventional explosions employed in the oil industry to locate sub-surface oil deposits. When an earthquake occurs, a sudden release of accumulated strain results in the propagation of a number of different types of seismological waves. Geophones have previously been employed to measure various parameters associated with earthquakes, such as the velocity of sub-surface movement of waves, the rate of change of the velocity, and the duration of the event. Shear waves, or S-waves, are the primary signal generated by an earthquake. When an extreme disturbance occurs in an homogeneous environment, a spherical front is generated, and a P-wave results. Such a wave is characterized by alternating compression and rarefaction through the sub-surface of the earth, somewhat similar to the structure of sound waves in air. Nuclear blasts in conjunction with underground tests primarily radiate P-waves.
At the interface of the earth with air Rayleigh waves are generated. Such waves are associated with both earthquakes and underground nuclear tests. Love waves are generated primarily from earthquakes, and are generally transverse to the direction of travel of Rayleigh waves. A wide variety of other complex waveforms resulting from reflection and refraction effects axe also known in seismology. A useful discussion of waves, along with recitation of the possibility of monitoring such waves for purposes of policing a total test ban treaty, is discussed in Scientific American, volume 247, No. 4, pages 47-55, October, 1982.
In the prior art a variety of geophones and/or seismometars have been proposed. Essentially known prior art devices include a rigid, generally conically shaped outer casing or enclosure housing an internal element of some form for sensing vibration. A variety of different sensors and/or transducers have been proposed to originate an electrical signal corresponding to seismological vibration. For example, Hayes in U.S. Pat. No. 1,980,993 discloses a sealed chamber in which pneumatic pressure results in the generation of an electrical signal in response to seismological vibration. Bound in U.S. Pat. No. 3,806,909 employs an internal piezoelectric element sensitive to soil stresses for generating a seismological responsive signal. Massa in U.S. Pat. No. 3,360,772 proposes a geophone in which a bilaminar piezoelectric element is suspended across an interior within the geophone housing for sensing vibrations and producing a proportional electrical signal.
The seismometer proposed by Baltosser in U.S. Pat. No. 2,748,370 contemplates the use of an electromagnetic sensor system interiorly of the casing for producing vibration sensing. Ording in U.S. Pat. Nos. 2,712,124 and 2,759,552 also discloses electromagnetic means for generating a proportional electrical signal. Sanderson in U.S. Pat. No. 2,677,270 senses vibration in response to the differential conductivity within a fluid medium as a gaseous bubble confined within a fluid chamber moves about in response to sudden seismological vibration. Other less relevant art known to me includes U.S. Pat. Nos. 2,683,867 and 3,474,405.
Seismological sensing technology relates generally to the broader science of perturbation monitoring. Physically shock resistive but sensitive motion sensors similar to the heart of the geophone in the aforedescribed copending patent application are of necessity in a wide variety of applications. Thus velocity meters, accelerometers, and servo-motion sensors commonly find usage within geophones, impact gauges, stethoscopes, inertial sensors, inertial guidance systems, vibration measuring systems, hydrophonic sensing instrumentation and the like.
A basic magnetohydrodynamic sensor embodies the capability of being useful in the three major modes of motion sensing (those being displacement meter, velocity meter, and accelerometer), either by direct transducer design or by servo-design.
Displacement meters are motion sensors whose natural period of vibration is larger than the period of frequency of the perturbation being measured. Such instruments indicate the actual linear displacement magnitude of the perturbation. Velocity meters are motion sensors whose signal output in response to perturbations is in direct proportion to the perturbation behaviour characteristics. Such instruments indicate the velocity of the motion being measured. Accelerometers are motion sensors which normally have natural periods of vibration which are shorter in duration than the frequency of incoming perturbations. Such sensors produce signal logic capable of measuring the acceleration of incoming measured perturbations. Velocity meters may be converted into accelerometers by the use of well known differentiator circuits. In the practice of seismology an earthquake-measuring instrumentation platform will commonly have all three types of motion sensors in use so that displacement, velocity, and acceleration can be logged concurrently.
Servo-motion sensors are designed to reduce the influence of mechanical losses during perturbation such as friction and "sloshing" of the working fluid medium inside of the sensors. By utilizing a small segment of electricity produced by the initial flow of the fluid within the device, the sensor mechanism has the capacity to utilize this electricity to trigger a counter-current which stops the flow of the fluid in the tube. In some cases, one can more accurately determine the actual amount of electrical current it takes to stop the flow of the liquid than one can accurately measure the direct flow of the fluid within the tube.
In the trade, velocity meters, accelerometers, and servo-motion sensors commonly find usage in such applications as geophones seismometers, impact gauges, stethoscope, inertial sensors as components of mutual guidance systems and various vehicle or vessel-steering or traverse-control systems, rotational motion measuring instruments, vibration measuring instruments, hydrophonic acoustical instruments, and other uses and application. Servo-sensors are omni-positional and work in the absence of gravitational field, which makes them particularly useful in space.